PHP, $this->{$var} -- what does that mean?

Question

I have encountered the need to access/change a variable as such:

$this->{$var} 

The context is with CI datamapper get rules. I can't seem to find what this syntax actually does. What do the {'s do in this context?

Why can't you just use:

$this->var 

Answer 1

This is a variable variable, such that you will end up with $this->{value-of-$val}.

See: http://php.net/manual/en/language.variables.variable.php

So for example:

$this->a = "hello"; $this->b = "hi"; $this->val = "howdy";  $val = "a"; echo $this->{$val}; // outputs "hello"  $val = "b"; echo $this->{$val}; // outputs "hi"  echo $this->val; // outputs "howdy"  echo $this->{"val"}; // also outputs "howdy" 

Working example: http://3v4l.org/QNds9

This of course is working within a class context. You can use variable variables in a local context just as easily like this:

$a = "hello"; $b = "hi";  $val = "a"; echo $$val; // outputs "hello"  $val = "b"; echo $$val; // outputs "hi" 

Working example: http://3v4l.org/n16sk

Answer 2

First of all $this->{$var} and $this->var are two very different things. The latter will request the var class variable while the other will request the name of the variable contained in the string of $var. If $var is the string 'foo' then it will request $this->foo and so on.

This is useful for dynamic programming (when you know the name of the variable only at runtime). But the classic {} notation in a string context is very powerful especially when you have weird variable names:

${'y - x'} = 'Ok'; $var = 'y - x'; echo ${$var};   

will print Ok even if the variable name y - x isn't valid because of the spaces and the - character.